It's been a while, but there was a paper or dissertation that discussed how to calculate robust ray offsets by pushing the low-order bits of the floats for the vector offset. I can't recall which paper or dissertation; can anyone please post a link?

FWIW the third edition of Physically Based Rendering has a new section on numerical robustness issues for ray tracing. There are (we think) some new contributions in it, so we've posted a PDF of that chapter in the hopes that people will take a look at it and see if there are any issues before the book goes off to the publisher: http://pbrt.org/fp-error-section.pdf.